420 The Rev. S. Haughton on Slaty Cleavage, 



1 



= 1 

 a 



I-' 



- =2-144 

 c 



(H) 



This result indicates an amount of compression quite com- 

 parable with that observed in the Ardoginna and Ardmore slates 

 of the same age, and may be considered as characteristic of the 

 carboniferous slates of the entire south of Ireland. 



It now remains to state how far the results I have obtained 

 correspond with those .arrived at by Mr. Sharpe. 



At the 87th page of the Quarterly Journal of the Geological 

 Society of London, Mr. Sharpe sums up the results of his obser- 

 vations as follows : — 



1st. In cleaved rocks there is a compression of the mass in a 

 direction everywhere perpendicular to the planes of cleavage. 



2nd. An expansion of the mass in the direction of the dip of the 

 cleavage. 



3rd. No proof has been found that the rocks have suffered any 

 change in the direction of the strike of the cleavage planes. 



Mr. Sharpe also mentions incidentally in the course of his 

 paper the following fact, of the importance of which he does not 

 seem to have been aware : — 



4th. When the plane of bedding is considered, the shells all 

 appear contracted in a direction perpendicular to the strike of the 

 cleavage plane across the bed. 



On comparing these results with the laws of cleavage distor- 

 tion given in the preceding part of this paper, it will be observed 

 that the first and fourth fall in with Laws III. and I. The 

 second and third statements do not accord with my observations, 

 and I am inclined to think that they are hasty generahzations, 

 as it would be extremely difficult to ascertain the absolute com- 

 pression or expansion undergone by the rock in any direction. 



It is evident that the whole subject of cleavage and its mecha- 

 nical origin is very imperfectly understood, and a desire to throw 

 even a little light upon so interesting a subject must be my excuse 

 for offering so imperfect a statement of the numerical results 

 which may be deduced from the measurement of the proportions 

 of the -distorted fossils which are found abundantly in many 

 highly cleaved rocks. 



It may be worth while to sum up the preceding results in the 

 form of a table, so as to admit of their easy comparison with 

 each other. 



